1. Field of the Invention
The present invention relates to an optical multiplexer/demultiplexer for use in optical wavelength (optical frequency) multiplex communication, and more specifically to an arrayed waveguide grating type optical multiplexer/demultiplexer having a sufficiently flat wavelength-dependent spectrum response in passing channel spacings or wavelength bands.
2. Discussion of the Background
In recent years, in the field of optical communication, researches have eagerly researched on an optical wavelength multiplex communication system that achieves a large increase in transmission capacity. In the multiplex communication system, the available transmission wavelength range is divided into a plurality of passing channel spacings, and pieces of information carried by a plurality of lightwave signals of different wavelengths are transmitted through a single optical fiber. For that purpose, an optical multiplexer/demultiplexer for multiplexing lightwaves of different wavelengths (frequencies) and for demultiplexing the wavelength-multiplexed lightwave into lightwaves of the original wavelengths is used in the optical wavelength multiplex communication.
To increase transmission capacity, it is effective to divide the transmission wavelength region into many passing channel spacings, or in other words, to use many lightwaves having narrow channel spacings. The optical multiplexer/demultiplexer is required to be capable of multiplexing and demultiplexing lightwaves having a frequency interval of 100 GHz, for instance, which corresponds to a wavelength interval of about 0.8 nm in the 1.55 xcexcm region.
In the optical wavelength multiplex communication, sometimes a relatively inexpensive LD light source is used to reduce the costs of constructing the communication system. However, the oscillating wavelength in the LD light source can deviate from the designed wavelength due to variations in ambient conditions, such as temperature and humidity, and can vary with time. Therefore, when an LD light source is used, the wavelength of lightwave to be multiplexed or demultiplexed can vary. On the other hand, the spectrum response of the optical multiplexer/demultiplexer in a passing wavelength band (channel spacing) is wavelength-dependent (spectrum response will be hereinafter referred to also as wavelength-dependent spectrum response).
Therefore, when the oscillating wavelength of the light source varies as mentioned above, the loss of the light passing through the optical multiplexer/demultiplexer varies depending on the spectrum response of the optical multiplexer/demultiplexer by the amount corresponding to the variation of the oscillating wavelength of the light source. Such variation of loss makes the loss of multiplexed/demultiplexed light ununiform between lightwaves having different wavelengths. This ununiformity causes a deterioration in a signal-to-noise ratio in the transmission of pieces of information carried by lightwave signals. The less flat the wavelength-dependent spectrum response of the optical multiplexer/demultiplexer is, the more ununiform the loss becomes.
As described above, the optical multiplexer/demultiplexer for use in an optical wavelength multiplex communication system is not only required to be able to multiplex and demultiplex many lightwaves of narrow channel spacings, but is also required to have a sufficiently flat wavelength-dependent spectrum response in passing channel spacings in the wavelength range used for the multiplex communication.
To meet the first requirement of multiplexing and demultiplexing lightwaves of narrow channel spacings, an optical multiplexer/demultiplexer using an arrayed waveguide grating has been proposed.
An arrayed wavelength grating type optical multiplexer/demultiplexer shown as an example in FIG. 8 has a plurality of input waveguides 2 formed on a substrate 1. The input waveguides 2 are connected to an end face 3a of an input-side slab waveguide 3 having the other end face 3b thereof connected to ends of one-side of a plurality of channel waveguides 4a that constitute an arrayed waveguide grating 4. The other ends of the channel waveguides 4a are connected to an end face 5a of an output-side slab waveguide 5 having the other end face 5b thereof connected to a plurality of output waveguides 6.
The input-side slab waveguide 3 has opposite end faces 3a, 3b. The end face 3b is formed to be a concave face that has a center of curvature positioned at the center of the other end face 3a. The end face 3a is formed to be a concave face having a center of curvature positioned at the middle point of a line connecting the centers of the end faces 3a, 3b. Similarly, the end face 5a of the output-side slab waveguide 5 is formed to be a concave face whose center of curvature is positioned at the center of the other end face 5b. The end face 5b is formed to be a concave face having a center of curvature positioned at the middle point of a line connecting the centers of the end faces 5a, 5b. 
As for the demultiplexer function of the optical multiplexer/demultiplexer, typically wavelength-multiplexed light is introduced through an input waveguide 2 connected to a central portion of, preferably, the center of the end face 3a of the input side-slab waveguide 3. From the input waveguide 2, the wavelength-multiplexed light is incident on the end face 3a of the input-side slab waveguide 3, and diffracted in the slab waveguide 3. Then, through the channel waveguides 4a that have different waveguide lengths, the light is incident on the end face 5a of the output-side slab waveguide 5, undergoes interference in the slab waveguide 5, and focuses on the other end face 5b of the slab waveguide 5. Focusing positions are different according to wavelengths. For example, lightwaves each having a central wavelength in a corresponding one of the passing channel spacings for the optical multiplexer/demultiplexer, focus on their respective focusing positions on the slab waveguide end face 5b and are taken out through the output waveguides 6 connected to those focusing positions, respectively. As for the multiplexer function, signal light beams entering the input waveguides 2 or the output waveguides 6 and having different wavelengths are multiplexed, and the multiplexed signal light is taken out from the input waveguide 2 or the output waveguide 6 connected to the center of the input or output side slab waveguide end face 3a or 5b. 
In the optical multiplexer/demultiplexer described above, the angular dispersion on the end face 5b of the output-side slab waveguide 5 is expressed as follows:
dxcex8/dxcex=m/(nsxc2x7d)xe2x80x83xe2x80x83(1)
In equation (1), xcex8 denotes the angle of diffraction, m is the order of diffraction, xcex is the wavelength of an input lightwave, ns is the refractive index of the slab waveguides 3 and 5, and d is the pitch between the channel waveguides 4a. 
When the focal length of the output-side slab waveguide 5 is denoted by F1, and the position on the slab waveguide end face 5b as viewed in the direction of the width of the slab waveguide 5 (typically, the distance from the center of the slab waveguide end face 5b to the focusing position) is denoted by x1, the linear dispersion on the end face 5b is expressed as follows:
dx1/dxcex=(mxc2x7F1)/(nsxc2x7d)xe2x80x83xe2x80x83(2)
As mentioned above, since the input-side slab waveguide 3 and the output-side slab waveguide 5 have the same focal length F1, the linear dispersion on the end face 3a of the input-side slab waveguide 3 is the same as the linear dispersion on the end face 5b of the output-side slab waveguide 5 expressed by equation (2).
Therefore, the electric field distribution of the light that focuses on the focusing position on the end face 5b (places at which the slab waveguide end face 5b and the output waveguides 6 are connected) corresponds to the electric field distribution of the input light incident on the input-side slab waveguide 3 from the input waveguide 2.
Generally, the broadening of the electric field distribution of the input light is relatively narrow. Therefore, in the conventional arrayed waveguide grating type optical multiplexer/demultiplexer, the broadening of the electric field distribution of the light, which focuses on the focusing position on the end face 5b of the output-side slab waveguide 5 and is taken out through the output waveguides 6 corresponding to those focusing positions, is relatively narrow. On the other hand, the broadening of the electric field distribution of the light observed at the focusing position closely relates to the flatness of the wavelength-dependent spectrum response in the passing wavelength band or channel spacing.
As mentioned above, the less flat the wavelength-dependent spectrum response of the optical multiplexer/demultiplexer is, the more ununiform, depending on wavelength, the passing loss of the light that passes through the optical multiplexer/demultiplexer becomes. Therefore, in the communication system using the conventional arrayed waveguide grating type optical multiplexer/demultiplexer that has insufficient flatness in a wavelength-dependent spectrum response, when light having a deviated central wavelength caused by the variation of the oscillating wavelength of the light source or the like passes through the multiplexer/demultiplexer, the loss of light becomes ununiform, which causes deterioration in the signal-to-noise ratio.
Accordingly, one object of the present invention is to provide an arrayed waveguide grating type optical multiplexer/demultiplexer having a desirable wavelength-dependent spectrum response in passing channel spacings.
To achieve the above and other objects, the present invention provides an arrayed waveguide grating type optical multiplexer/demultiplexer in which a diffraction grating side end face of an input-side slab waveguide and a diffraction grating side end face of an output-side slab waveguide are connected by an arrayed waveguide grating including of a plurality of channel waveguides. In addition, at least one input waveguide is connected to a waveguide end face of the input-side slab waveguide, and at least one output waveguide is connected to a waveguide end face of the output-side slab waveguide. The optical multiplexer/demultiplexer also has a focal length of the diffraction grating end face of the output-side slab waveguide which is longer than a focal length of the waveguide end face of the input-side slab waveguide.
The optical multiplexer/demultiplexer of the present invention, which includes an arrayed waveguide grating capable of multiplexing lightwaves of narrow channel spacings and demultiplexing a wavelength-multiplexed lightwave made of such lightwaves, is suited to multiplex and demultiplex lightwave signals in an optical wavelength (optical frequency) multiplex communication system using, for signal transmission, the transmission wavelength region divided into a plurality of passing channel spacings. Specifically, since the focal length of the output-side slab waveguide end face is longer than the focal length of the input-side slab waveguide end face, the broadening of the electric field distribution of the light observed on the output-side slab waveguide end face is broader than the broadening of the electric field distribution of the light on the input-side slab waveguide end face. This improves the flatness of the wavelength-dependent spectrum response in each passing channel spacing for the optical multiplex communication. In the optical communication system, when the light whose central wavelength is deviated due to variation of the oscillating wavelength of a light source or the like passes through an optical multiplexer/demultiplexer, the loss of light can be varied by the deviation of wavelength. The variation of the loss of light can cause a deterioration in a signal-to-noise ratio in the communication system. On the other hand, according to the optical multiplexer/demultiplexer of the present invention having a sufficiently flat wavelength-dependent spectrum response, the wavelength-dependency of the loss of the light passing through the multiplexer/demultiplexer is reduced as long as the deviated central wavelength falls within a region where the wavelength-dependent spectrum response is flat. Thus, the variation of the loss of light is reduced, and the deterioration in signal-to-noise ratio is reduced.
More specifically, in the optical wavelength multiplex communication, it is desirable that the loss of the light passing through an optical multiplexer/demultiplexer and observed at a wavelength falling within a channel spacing is not larger than that observed at the central wavelength of the channel spacing, for example, by 1 dB or more. In other words, it is desirable for the optical multiplexer/demultiplexer to have a broad 1 dB passing band width (1 dB channel spacing width). The present invention provides an optical multiplexer/demultiplexer that meets such a requirement, and therefore is of significant help in constructing an optical wavelength multiplex communication system.